With this type of sensor unit, by fixing the circuit board on which the sensor is mounted to the housing, the sensor unit can be integrally handled. Assembling errors are unavoidable between the sensor, circuit board and housing. Therefore, many sensors are configured to correct detection signals using initial calibration parameters for correcting measurement errors resulting from assembling errors. The initial calibration parameters are unidentifiable unless measurement is carried out by completing the measuring system. Thus, initial calibration is carried out in which the detection signals of the sensor are transmitted to an external calibration device, and initial calibration parameters calculated in the external calibration device are written in a non-volatile semiconductor memory of the sensor (see e.g. JP patent publication 2004-191101A).
In order to reduce input lines to the sensor, one known sensor of this type is configured to write the initial calibration parameters with a calibration voltage that is higher than the working voltage being supplied to a single input end of the sensor, and then correct the detection signals using initial calibration parameters stored in a non-volatile memory, with working voltage supplied to the single input end of the sensor.
But in the above arrangement, in which the working voltage or calibration voltage is supplied through the single input end of the sensor, it is impossible to mount a voltage stabilizing the voltage value of the supply power source to the working voltage between the input end of the sensor and a power source connecting end for the sensor on the circuit board. This is because it is impossible to supply the calibration voltage to the input end of the sensor. But if the voltage stabilizing circuit is not mounted, it becomes difficult to use the sensor unit if the power source voltage is unstable according to the use environment, such as if a power source voltage is supplied from a generator attached to the driving source of e.g. a construction machine or an industrial machine, or if the voltage value of the supply voltage cannot be identified as a single value.